Known in the art is a spark ignition type internal combustion engine provided with a variable compression ratio mechanism able to change a mechanical compression ratio and a variable valve timing mechanism able to control a closing timing of an intake valve, performing a supercharging action by a supercharger at the time of engine medium load operation and engine high load operation and, in the state holding an actual compression ratio fixed at the time of engine medium and high load operation, increasing the mechanical compression ratio and retarding the closing timing of the intake valve as the engine load becomes lower (for example, see Japanese Patent Publication (A) No. 2004-218522).
However, in this internal combustion engine, even at the time of engine low load operation, the mechanical compression ratio is made high and the closing timing of the intake valve is retarded. However, how the amount of intake air fed into the combustion chamber is controlled at the time of engine low load operation is unclear.
Further, generally speaking, in an internal combustion engine, the lower the engine load, the worse the heat efficiency, therefore to improve the heat efficiency at the time of vehicle operation, that is, to improve the fuel efficiency, it becomes necessary to improve the heat efficiency at the time of engine low load operation.
However, in an internal combustion engine, the larger the expansion ratio, the longer the period in an expansion stroke where a downward force acts on the piston, therefore the larger the expansion ratio, the more the heat efficiency is improved. Contrarily, the higher the mechanical compression ratio, the larger the expansion ratio. Therefore to improve the heat efficiency at the time of vehicle operation, it is preferable to raise the mechanical compression ratio at the time of engine low load operation as much as possible to enable the maximum expansion ratio to be obtained at the time of engine low load operation.
Further, in an internal combustion engine, as the opening degree of the throttle valve becomes smaller, the pumping loss becomes larger. This tendency becomes remarkable at the time of engine low load operation. If the pumping loss becomes larger in this way, the heat efficiency drops. Therefore, to improve the heat efficiency at the time of vehicle operation, it is necessary to increase the opening degree of the throttle valve so reduce the pumping loss at the time of engine low load operation. In this way, to increase the opening degree of the throttle valve at the time of engine low load operation, that is, when the amount of intake air to be fed to the combustion chamber is small, it is preferable to change the closing timing of the intake valve in order to control the amount of intake air.
However, there is a limit to changing the closing timing of the intake valve to control the amount of intake air. When the engine load is extremely low, that is, when the amount of intake air to be fed into the combustion chamber is extremely small, it soon becomes difficult to suitably control the amount of intake air by retarding the closing timing of the intake valve.